The invention concerns a time/analog converter for the output of a standardised output voltage, for example for a magnetostrictive position sensor. The invention further concerns a method of setting a zero point and amplifying the output characteristic of a sensor such as an ultrasonic position sensor.
A magnetostrictive position sensor involves the excitation of electrical current pulses, resulting in the generation of magnetoelastic position pulses which are propagated along a ferromagnetic waveguide. Displacement of a position magnet along the measurement region of the position sensor causes a change in the period of time until the response signal is received. The time interval tx between excitation of the current pulse (START) and reception of the magnetoelastic position pulse (STOP) at an end of the waveguide is a measurement in respect of the spacing x of a position magnet from that location.
In general an analog voltage is to be provided as the position value at the output of the magnetostrictive position sensor. For that purpose the time interval tx has to be proportionally converted into a voltage as follows:
Uout (tx)=txxc2x7axc2x7Uref; with to less than t less than t1xe2x80x83xe2x80x83(1)
In the above equation tx denotes the signal transit time which is dependent on the position of the position magnet, Uref denotes the voltage of a reference voltage source of adequate accuracy and stability, and a denotes an imaging factor for representation from a time interval to a voltage interval. The design configuration of a magnetostrictive sensor with a detection portion at one end of the waveguide thereof means that it is not possible for the entire length of the waveguide along which the position pulse passes to be used as a measurement range. In addition, for the purposes of adaptation to various measurement ranges, respectively different time interval ranges have to be mapped on to the same output voltage range of preferably between 0V and 10V.
The purpose of a time/analog converter in this context is to prepare for different measurement ranges a standardised output voltage of preferably between 0V and 10V, as is required for further processing by a sequential circuit, in other words, the non-standardised input characteristic which is generally a straight line is to be deformed and shifted in such a way that it entails the standardised output values of 0V and 10V at the respective end positions of the measurement range (which end positions can be denoted by references tx0, tx1. For that purpose the time intervals of the position sensor, which are formed by the START and STOP signals, must be scaled to the desired standardised output voltage interval, for example as indicated above between 0V and 10V. That is effected in the following form:
Uout (tx)=Urefxc2x7axc2x7(txxe2x88x92b) orxe2x80x83xe2x80x83(2)
Uout (x)=Uref (axe2x80x2xc2x7xxe2x88x92bxe2x80x2)xe2x80x83xe2x80x83(3)
The scaling procedure is implemented by means of suitably selected parameters denoted by a meaning amplification or gain and b denoting zero point correction. The time interval tx formed by the sensor by virtue of the START and STOP pulses is converted into a standardised output voltage of preferably between 0V and Uref or 10V.
It is known for such a measurement conversion operation to be executed by means of integrating time/analog converters referred to often for the sake of brevity as T/A-converters which use integrators with holding members. Such T/A-converters provide that the integration time constant of the integrator or the gain of a downstream-connected amplifier stage is altered by means of trimming potentiometers. That involves the disadvantage that manual adjustment is necessary and thus automation of the scaling procedure is not an option.
An object of the present invention is to provide a time/analog converter for providing a standardised output voltage, such as for a magnetostrictive position sensor, which can provide for scaling automatically or by programming and which in particular does not require a trimming potentiometer.
Another object of the present invention is to provide a time/analog converter for adjusting zero point and amplifying output characteristic of a sensor such as a magnetostrictive position sensor, which while being of a simplified design configuration affords reliable operational results.
Still a further object of the present invention is to provide a method of setting a zero point and amplifying an output characteristic of a sensor such as an ultrasonic position sensor, which comprises a more rational sequence of operating steps and which affords a reliably attainable procedural result.
In the time/analog converter aspect in accordance with the principles of the present invention the foregoing and other objects are attained by a time/analog converter for adjusting the zero point and amplifying the output characteristic of a sensor such as a magnetostrictive position sensor, comprising an integrator having an output connected by way of a switch to an input of a holding member. The output voltage of the holding member and a reference voltage are applied to the input of the integrator by way of further switching means. Preferably each input voltage of the integrator is applied to a separate switch. The converter further includes a control means such as a microcontroller for actuating the switches in such a way that the reference voltage is conducted to the input of the integrator for a predetermined first time interval and the output voltage of the holding member is conducted to the input of the integrator for a predetermined second time interval, the control means being operable to actuate the first-mentioned switch in such a way that the output voltage of the integrator is applied to the holding member for a predetermined third time interval. A controllable pulse shaper is connected to the switching connection of one of the separate switches of the switching means and can close same for a desired time interval.
In the method aspect the foregoing and other objects are attained by a method of setting a zero point and amplifying the output characteristic of a sensor, for example an ultrasonic position sensor, comprising setting time scaling parameters which can typically be time intervals to a defined basic position, placing the sensor at a first position and measuring the output value thereof. The output value can then be trimmed to a predetermined first value by calibration of a first of the scaling parameters. The sensor is thereupon placed at a second position and the output value thereof measured, followed by trimming of the output value to a predetermined second value by calibration of a second of the scaling parameters.
As will be seen in greater detail from the description hereinafter of preferred embodiments of the invention, the control means actuates the switches in such a fashion that the output voltage of the holding member is conducted for a predetermined time interval identified as T and the reference voltage is conducted for an unknown time interval identified as txxe2x88x92toff. It is then that the control means switches on a third switch so that the output voltage of the integrator is applied to the input of the holding member for a predetermined third time interval.
For the integrator with an integration time constant xcfx84=RC over the time interval ton=[toff; tx] and T, the output voltage of the integrator is:                                           Uout            l                    =                                    Uout              o                        -                                          ∫                toff                tx                            ⁢                                                (                                      Uref                    ⁢                                          /                                        ⁢                    τ                                    )                                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  t                                                      -                                          ∫                toff                                  toff                  +                  T                                            ⁢                                                (                                                            Uout                      to                                        ⁢                                          /                                        ⁢                    τ                                    )                                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  t                                                                    ,                            (        4        )            
wherein Uout0 and Uout1 denote the integration output voltage before and after a measurement cycle respectively, toff denotes the delay time after the reception of a START-signal, tx denotes the time interval between the START-signal and the associated STOP-signal and to is the time at which the first switch is closed, which is generally simultaneous with the START-signal. That gives the differential equation with:
Uouti+1=Uouti+Uref (txxe2x88x92toff)/xcfx84xe2x88x92Uouti T/xcfx84xe2x80x83xe2x80x83(5)
and for the steady-state condition with:
Uouti+1=Uouti=Uout (x)xe2x80x83xe2x80x83(6)
that gives
Uout(x)=Urefxc2x71/T(txxe2x88x92toff)xe2x80x83xe2x80x83(7)
wherein T is the integration cycle time with which the returned output parameter is involved in the integral. A comparison with equation (4) gives the amplification or gain axe2x80x2 with 1/T and zero point correction bxe2x80x2 with toff/T.
The length of the time interval toff, T is determined by the programmable control means, preferably, as indicated above, a microcontroller. The first scaling parameter toff which determines the offset of the output characteristic is formed by the control means preferably by the delay time of a second delayed start signal STARTxe2x80x2 which is applied to a bistable trigger circuit or multivibrator. The first switch then remains closed for a period ton with ton=[toff; tx], until a STOP-signal is received.
The second scaling parameter T which determines the gradient of the output characteristic of the sensor is also generated by the control means. For the purposes of trimming the parameter T, it is in principle possible to use any controllable timer which permits adequate fine trimming. In accordance with the present invention however a controllable pulse shaper is used, which is automatically adjustable and which has a fine resolution, with which it is also possible to compensate for tolerances in the sonic transit time of the waveguide.
Adjustment of the time interval toff, T is preferably effected under the control of a test unit such as a test field computer which is connected to the control means by way of its programming interface and at which, for given measurement positions of the sensor, trimming of the output signal of the T/A-converter is implemented in the desired manner, by suitably adjusting the scaling parameters toff, T.
Parameterisation of the system, that is to say adjustment of zero point and amplification or gain of the output characteristic of the position sensor, can be effected at a plurality of measurement points in the measurement range of the sensor. In a preferred feature however parameterisation is effected only at first and second measurement points of the sensor, in which respect the sensor is preferably moved only once from the first measurement position to the second measurement position.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of preferred embodiments.